1. Technical Field
The invention relates generally to providing a combination of cap and plastic container that provides a snug fit while remaining easily removable. More specifically, the invention relates to providing an inexpensive, injection molded cap for an inexpensive, blow molded container that nevertheless provides a good seal.
2. Related Art
In offering food products to the consumer, convenience and cost are two considerations that receive a lot of attention. This applies not only to the food product itself, but also to the packaging in which it is marketed. The vast majority of products are either wrapped in a plastic film or provided in a disposable container. If the product is packaged in a quantity greater than a single serving, there may be both an original seal, designed to seal in freshness and offer evidence of tampering, as well as an overcap used to re-close the package between uses.
Thin, plastic snap-on caps are often used to provide closure for disposable food containers once a sealing closure has been removed. FIG. 1 shows a perspective of a prior art container 110 and overcap 120 that can be used for food items. When the product is initially placed in the container 110, a freshness seal 130 is placed over the opening to the container 110 and fixed there, such as by an adhesive. An overcap 120 is then placed on the container 110 over the freshness seal 130. When the consumer is ready to consume the product, they will remove both the overcap 120 and freshness seal 130 to consume the product. The freshness seal 130 will be disposed of, but the overcap 120 is typically retained to provide a closure to protect remaining product.
Injection molding can be used to make the overcaps inexpensively. Examples of containers on which these are used include paperboard containers having a plastic or metal rim (used, for example, with oatmeal or roasted nuts) and plastic tubs (for soft cheeses and butter). Typically, the overcap 120 has a rounded ridge 122 on the inside, which snaps over a similar ridge 112 on the container 110. In some cases, the fit of the cap to the container is not a prime concern, as the product does not quickly stale, such as with butter. When maintaining freshness is important, such as with products that stale quickly, a tight seal of overcap to container is desirable. In these applications, the container is typically made of a heavier material, such as paperboard, and often the rim of the container is made of a material, such as a metal, for which the manufacturing tolerances are small. The downside of this approach is the cost, as these techniques are more expensive than molded plastic.
Blow molding is a commonly used technique for forming thin-walled plastic containers. In one version of this molding technique, a thick-walled tube of plastic (shaped similarly to a test tube) is first heated and placed inside a mold. The tube is then inflated by injecting air into it, so that the tube expands to fit the inside of the mold. The mold is chilled to cool the plastic quickly. Blow molding techniques have made inexpensive containers possible, although it is not possible to meet tight tolerances with just blow molding. When a blow-molded bottle needs a tight lid, e.g., for soft drinks, the neck of the bottle is formed by another technique, allowing a tighter fit to the lid.
Because blow molding a container and injection molding a snap-on cap are inexpensive methods of producing a lidded container, it would be desirable to manufacture a lidded container by these processes. However, it is difficult to produce an injection molded snap-on cap to fit the variations that can be produced by blow molding a container. FIG. 2a shows a prior art combination as it is designed to fit. FIG. 2b demonstrates the problem of a loose fit when injection molded cap 220 is at the large end of its tolerance and the blow molded container 210 is at the small end of its tolerance. In this case, the cap can be easily pushed off, even by excess pressure within the container. FIG. 2c demonstrates the problem at the other extreme of the fit spectrum, where the injection molded cap 220 is at the small end of its tolerance and the blow-molded container 210 is at the large end of its tolerance. In this instance, the cap can fit so snugly that it is difficult to remove. Additionally, there is commonly only a single point of contact between the container and cap when viewed in cross-section. This does not provide the seal that is necessary when the product degrades under prolonged exposure to the air.
Of course, many different shapes of lid and containers are possible. For instance, FIGS. 3a and 3b demonstrate a number of prior art lids and their ideal fit to a corresponding container. FIG. 3a is taken from U.S. Pat. No. 6,047,851 to Freck et al. Freck's container has a rounded edge to act in place of a rounded bead and the patent is directed to modifying that edge from a prior art shape to better allow the cap to be removed without cracking. The cap of Freck is apparently intended to fit snugly against the container across most of the rim of the container. FIG. 3b is taken from U.S. Pat. No. 3,892,351 to Johnson et al. The tubular container is a glue-bonded, paperboard composite, spirally wound tube, with its top rim rolled outwardly to form a circumferentially extending bead. The overcap has a radially inwardly and downwardly extending shoulder that engages with the rolled rim of the container.
In order to provide an inexpensive method of packaging snack foods, it would be desirable to design a better snap-on overcap that can be used with a blow-molded container in order to provide packaging for a snack product. Since packaging for such a product is considered a disposable, it is desirable to keep the costs of such a combination container/overcap low. At the same time, although it is not necessary for the overcap to protect the product during shipping, it should be sufficiently well fitting that the product remaining after an initial opening of the container can be protected from absorbing too much moisture, which can cause degradation of the product.